White LEDs are conventionally expected to be applicable for illumination purpose as next-generation light sources to replace incandescent bulbs and fluorescent lamps. LED devices, such as white LED devices, generally have a structure in which an excitation LED chip is covered with a wavelength conversion member, or a mixture of an inorganic phosphor powder and a resin, by molding. However, heat and light emitted from such an LED chip are collectively applied to a limited region of the molded cover, and thus the resin used for the molded cover may often be colored or deformed in a short time. Therefore, there arises a problem in that a change of the luminescent color occurs in a short time to shorten the lifetime of the LED device. It is considered that this problem will become more serious with increasing output power of LED chips. Hence, there has been a demand to develop a wavelength conversion member having excellent heat resistance.
To address this problem, a wavelength conversion member has been proposed which is made of a fully inorganic solid without using any resin (see, for example, Patent Document 1). Since this wavelength conversion member does not use any resin having poor heat resistance and is made of a fully inorganic solid, it has excellent heat resistance and thus hardly causes thermal degradation.    Patent Document 1: Published Japanese Patent Application No. 2003-258308